diff --git a/lib/logic/connection_settings.dart b/lib/logic/connection_settings.dart
index 6189360..658e9c2 100644
--- a/lib/logic/connection_settings.dart
+++ b/lib/logic/connection_settings.dart
@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 enum ConnectionMode { tun, proxy }
@@ -18,23 +19,37 @@ abstract final class ConnectionSettings {
     final prefs = await SharedPreferences.getInstance();
     final storedMode = prefs.getString(_modeKey);
 
-    // Any unrecognized value (including the pre-V3 'auto') is TUN. 'proxy' is
-    // the only persisted value that selects the SOCKS5 mode.
+    // Only the explicit current proxy value restores SOCKS5. Unknown and
+    // obsolete values, including the pre-V3 'auto' label, fail closed to
+    // full-tunnel TUN instead of silently selecting a weaker route.
     _mode = storedMode == ConnectionMode.proxy.name
         ? ConnectionMode.proxy
         : ConnectionMode.tun;
 
     if (storedMode != _mode.name) {
-      await prefs.setString(_modeKey, _mode.name);
+      // Best-effort. _mode is already the safe TUN default in memory, so a
+      // failed write leaves nothing insecure — it only means the migration
+      // is retried next launch. initialize() runs from main() before
+      // runApp(), with no zone guard, so throwing here would black-screen
+      // the app on the fresh-install path for no security gain.
+      final migrated = await prefs.setString(_modeKey, _mode.name);
+      if (!migrated) {
+        debugPrint('[ConnectionSettings] Could not persist migrated mode.');
+      }
     }
 
     _initialized = true;
   }
 
-  static Future<void> setMode(ConnectionMode mode) async {
+  static Future<bool> setMode(ConnectionMode mode) async {
+    if (_initialized && _mode == mode) return true;
+
+    final prefs = await SharedPreferences.getInstance();
+    final saved = await prefs.setString(_modeKey, mode.name);
+    if (!saved) return false;
+
     _mode = mode;
     _initialized = true;
-    final prefs = await SharedPreferences.getInstance();
-    await prefs.setString(_modeKey, mode.name);
+    return true;
   }
 }
diff --git a/lib/screens/settings/in_settings/connection_mode.dart b/lib/screens/settings/in_settings/connection_mode.dart
index 021c3b0..46519d7 100644
--- a/lib/screens/settings/in_settings/connection_mode.dart
+++ b/lib/screens/settings/in_settings/connection_mode.dart
@@ -51,8 +51,14 @@ class _ConnectionModeTileState extends State<ConnectionModeTile> {
       return;
     }
 
-    await ConnectionSettings.setMode(next);
+    final saved = await ConnectionSettings.setMode(next);
     if (!mounted) return;
+    if (!saved) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('Could not save connection mode.')),
+      );
+      return;
+    }
 
     setState(() {
       _mode = next;